A reactor of this type comprises a core consisting of fuel arrays of two different types, arranged side by side. Some of them, referred to as "fissile arrays", mainly contain a fissile material, whereas the others, referred to as "fertile arrays", contain a material which is capable of being converted to fissile material under the action of neutron bombardment. The fertile arrays are generally arranged at the periphery of the core, where they collect the neutron radiation produced by the fissile arrays.
The reactor core is immersed in a moderating fluid, such as water, which generally also serves as a heat-transfer fluid. This fluid circulates in contact with the fuel elements arranged inside the arrays.
Compared with pressurized water nuclear reactors of the conventional type, which only comprise fissile arrays, the undermoderated reactor has a very much lower ratio of volume of moderator/volume of fissile material in the core, or moderating ratio VM/VU, whereby the energy spectrum of the neutrons is very different.
This spectrum is intermediate between that of a conventional pressurized water reactor, in which the neutrons are very slow, and that of fast reactors, in which the neutrons are not slowed down.
The energy spectrum of the neutrons in undermoderated reactors is said to be "epithermal".
This epithermal spectrum of neutrons makes it possible to produce fissile material from fertile material arranged, for example, at the periphery of the core. A low moderating ratio is obtained, in so-called "undermoderated" reactors, by making provision for a distribution of the fuel material in the fissile arrays which ensures optimum contact with the moderator and at the same time a low volume ratio of the moderator to the fissile material.
Whether fissile or fertile, the fuel arrays generally consist of a prism-shaped casing made of a weakly neutron-absorbing material, such as a zirconium alloy, inside which long tubular rods, containing the fissile material or the fertile material, are arranged parallel to the height of the casing and in a uniform lattice in the cross-sections of this casing. These rods of fuel material consist of a tube made of canning material, filled with pellets containing the fissile or fertile material. These rods, arranged parallel to one another in the array, form a bundle, the spacing between the rods being maintained by spacer devices, such as wires wound in a helix around the can of the rods, or ribs and roughnesses provided on this can. Such arrangement enables the heat-transfer and moderating fluid to circulate in contact with the whole surface of the rod while permitting a very small spacing between the rods, i.e., that it makes it possible to obtain a very dense lattice in which, however, the cans of the rods are never contiguous. This high-density lattice of rods of fissile material makes it possible to obtain a low moderating factor.
It is nevertheless difficult to ensure a constant moderating level throughout the cross-section of a fuel array for an undermoderated nuclear reactor; in particular, in the region of the peripheral zones of the array, the moderating level varies considerably because of the presence of a relatively thick sheet of water in the region of the wall in the zone between two successive rods.
This can result in a deterioration of the performance characteristics of undermoderated reactors.